On 01/02/11 17:02, Keld Jørn Simonsen wrote:
On Tue, Feb 01, 2011 at 11:01:33AM +0100, David Brown wrote:
On 31/01/2011 23:52, Keld Jørn Simonsen wrote:
raid1+0 and Linux MD raid10 are similar, but significantly different
in a number of ways. Linux MD raid10 can run on only 2 drives.
Linux raid10,f2 has almost RAID0 striping performance in sequential read.
You can have an odd number of drives in raid10.
And you can have as many copies as you like in raid10,
You can make raid10,f2 functionality from raid1+0 by using partitions.
For example, to get a raid10,f2 equivalent on two drives, partition them
into equal halves. Then make md0 a raid1 mirror of sda1 and sdb2, and
md1 a raid1 mirror of sdb1 and sda2. Finally, make md2 a raid0 stripe
set of md0 and md1.
I don't think you get the striping performance of raid10,f2 with this
layout. And that is one of the main advantages of raid10,f2 layout.
Have you tried it out?
No, I haven't tried it yet. I've got four disks in this PC with an
empty partition on each specifically for testing such things, but I
haven't taken the time to try it properly.
But I believe you will get the striping performance - the two raid1
parts are striped together as raid0, and they can both be accessed in
parallel.
As far as I can see the layout of blocks are not alternating between the
disks. You have one raid1 of sda1 and sdb2, there a file is allocated on
blocks sequentially on sda1 and then mirrored on sdb2, where it is also
sequentially allocated. That gives no striping.
Suppose your data blocks are 0, 1, 2, 3, ... where each block is half a
raid0 stripe. Then the arrangement of this data on raid10,f2 is:
sda: 0 2 4 6 .... 1 3 5 7 ....
sdb: 1 3 5 7 .... 0 2 4 6 ....
The arrangement inside my md2 is (striped but not mirrored) :
md0: 0 2 4 6 ....
md1: 1 3 5 7 ....
Inside md0 (mirrored) is then:
sda1: 0 2 4 6 ....
sdb2: 0 2 4 6 ....
Inside md1 (mirrored) it is:
sdb1: 1 3 5 7 ....
sda2: 1 3 5 7 ....
Thus inside the disks themselves you have
sda: 0 2 4 6 .... 1 3 5 7 ....
sdb: 1 3 5 7 .... 0 2 5 7 ....
I don't think there is any way you can get the equivalent of raid10,o2
in this way. But then, I am not sure how much use raid10,o2 actually is
- are there any usage patterns for which it is faster than raid10,n2 or
raid10,f2?
In theory raid10,o2 should have better performance on SSD's because of
the low latency, and raid10,o2 doing multireading from each drive, which
raid0,n2 does not.
I think it should beat raid10,n2 for some things - because of
multireading. But I don't see it being faster than raid10,f2, which
multi-reads even better. In particular with SSD's, the disadvantage of
raid10,f2 - the large head movements on writes - disappears.
We lack some evidence from benchmarks, tho.
Indeed.
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